1. Field of the Invention
This invention relates to an image projection device of the type known as an xe2x80x9cimage floater,xe2x80x9d in which an illuminated image is projected by optical elements to a position between the optical elements and an observer, the projected image appearing to float in free space.
More specifically, the invention relates to an image projection device which uses a Fresnel lens system to cause light rays from a source image to converge in front of the Fresnel lens and form a real image at the point of convergence. Similar image floaters are disclosed in U.S. Pat. Nos. 4,671,625 and 5,782,547, herein incorporated by reference.
The invention improves upon the image projection devices disclosed in U.S. Pat. Nos. 4,671,625 and 5,782,547 by replacing the pairs of Fresnel lenses disclosed in the prior patents with (i) a single Fresnel lens, and (ii) at least one light guiding element arranged to cause light rays from the image source to pass through the Fresnel lens once in each direction. By causing light rays from the image source to pass through the Fresnel lens twice, the single Fresnel lens performs the same functions as the respective pairs of lenses in the prior image projection devices, thereby simplifying the structure of the prior image projection devices without loss of performance.
2. Description of Related Art
xe2x80x9cImage floatersxe2x80x9d or image projection devices that project an image so that it appears to float in free space have been well known since at least the beginning of the last century. One way to achieve the image floater effect is to arrange optical elements such as parabolic mirrors or concave lenses so that they cause light rays originating from the source image to converge at a location between the optical element and the viewer, thereby forming a xe2x80x9crealxe2x80x9d at the location of convergence.
The present invention is an improvement to the image floaters disclosed in U.S. Pat. Nos. 4,671,625 and 5,782,547. Each patent proposes to replace the lenses and mirrors of earlier xe2x80x9cimage floatersxe2x80x9d by a Fresnel lens system consisting of two positive focal length Fresnel lenses.
The principles of operation of the Fresnel image floaters are shown in FIG. 1. The illustrated Fresnel image floater includes two positive focal length Fresnel lenses which may, but are not required to be, identical in construction. The first positive focal length Fresnel lens 1 of the conventional arrangement is used as a collimator to direct light rays from points on a source image 2 to a second positive focal length Fresnel lens 3. The second positive focal length Fresnel lens 3 collects the light rays transmitted by the first positive focal length Fresnel lens and focuses them in front of the second Fresnel lens. The two lenses thus serve to direct and cause light rays from the source image to converge at locations in front of the second lens, so that a real image 4 appears in front of the second lens.
Although illustrated in FIG. 1 as a linear system, the Fresnel lens image floaters disclosed in U.S. Pat. Nos. 4,671,625 and 5,782,547 can actually have a wide variety of configurations, achieved by varying the path taken by light rays from the image source to the first Fresnel lens, and by varying the path of converging light rays exiting the second Fresnel lens. For example, as shown in FIG. 2, an optical component such as mirror 5 may be positioned to reflect light exiting the second Fresnel lens 3 to a location outside image floater. In general, mirrors lenses, and other optical elements or components may be positioned anywhere in the path of light from the image source to the location of the projected image, so as to vary the image path and accommodate different source image orientations and/or to change the position of the projected image.
Not only can the positions of the image source and projected image be varied by changing the path of light entering and exiting the Fresnel lens system, but additional images can be superimposed on the projected image. For example, as shown in FIG. 2, the light guiding mirror 5 may be a half-silvered or partially transmissive mirror, in which case mirror 5 serves as a beam splitter to combine a background image 6 with the projected image 4. U.S. Pat. No. 5,782,547, in particular, describes a number of ways to enhance such a background image by adding the illusion of depth to the background image, thereby complementing the three dimensional appearance of the projected image.
The present invention is in principle applicable to all of the various image floaters disclosed in U.S. Pat. Nos. 4,671,625 and 5,782,547, as well as to any other existing dual lens Fresnel image floater devices, differing from the prior arrangements solely in that it replaces the pair of lenses 1,3 shown in FIG. 1 with a Fresnel lens that performs a collimating function in one direction and a focusing function in the second. This is made possible by the property of Fresnel lenses (and other convex, positive focal length lenses) that the collimating lens can be a mirror image of the focusing lens, i.e., lens 1 shown in FIG. 1 is identical to lens 3, the only difference being the side on which the light rays are incident.
The xe2x80x9csinglexe2x80x9d Fresnel lens arrangement of the present invention should not be confused with arrangements in which one of the two Fresnel lenses of the Fresnel image floater is simply eliminated without modifying the light path, so that light rays from a source image only pass once, rather than twice, through the remaining Fresnel lens. U.S. Pat. No. 5,782,547, for example, mentions such an arrangement in which the image floater is xe2x80x9cconfigured without a second Fresnel lens.xe2x80x9d While the function of a deleted Fresnel lens can be emulated by optical elements such as concave mirrors or focusing lenses, such arrangements are nearly always disadvantageous because the elements that replace the Fresnel lens are likely be more complex, expensive, and/or less accurate than the Fresnel lens itself.
In contrast, in the present invention, a light ray passing twice through the Fresnel lens effectively xe2x80x9cseexe2x80x9d two Fresnel lenses, and thus the light ray is affected in exactly the same manner as in the dual-Fresnel image floaters, and without having to add any optical elements other than relatively simple (in comparison with concave mirrors and the like) guiding elements arranged to direct light rays transmitted by the Fresnel lens back through the lens. Thus, although U.S. Pat. No. 5,782,547 mentions an embodiment xe2x80x9cconfigured without a second Fresnel lens,xe2x80x9d the embodiment without the second Fresnel lens is not analogous to the single Fresnel lens arrangement of the present invention, and is likely to result in a significant loss of brightness in the projected image.
Another image floater arrangement which utilizes only a single Fresnel lens and yet is not analogous to the image floater of the invention is disclosed in U.S. Pat. No. 5,944,403. In this arrangement, a concave mirror causes rays of light to start to converge before passing through the single Fresnel lens. Again, the light rays are not caused to pass through the Fresnel lens twice, once in each direction. In addition, the image floater device of U.S. Pat. No. 5,944,403 suffers the disadvantages that the concave mirror is expensive and difficult to align, and that the resulting image is a virtual rather than real image, so that the image appears to float behind the Fresnel lens, limiting the ability of the image to be projected outside the image floater device.
In summary, the present invention represents an improvement to the image projection devices disclosed in U.S. Pat. Nos. 4,671,625 and 5,782,547. The dual Fresnel lens arrangement disclosed in these patents is replaced by a Fresnel lens and light guide arranged to guide light through the Fresnel lens in two directions. Unlike other single Fresnel systems which do not provide for passage of light through the Fresnel lens in two directions, the Fresnel lens system of the invention offers identical performance to a dual lens Fresnel arrangement, as well as equivalent versatility, including the ability to vary the orientation and positions of the image source and projected image, the ability to add a background image with any desired image enhancements, and the ability to cascade multiple image floaters so as to provide series of images floating in free space at different positions.
It is accordingly a first objective of the invention to provide a simple and compact image projection device of the type in which an image is projected in front of the image projection device and appears to float in free space.
It is a second objective of the invention to provide an image floater having a simple and compact construction, in which the projected image can be formed anywhere inside or outside the device, which easily permits the addition of background images, and which also permits cascading of multiple image floaters to provide multiple floating images.
It is a third objective of the invention to provide an image floater that performs in exactly the same manner as conventional dual lens image floaters, has all of the versatility, and is at least as easy to assemble, low in cost, and reliable as the prior image floaters, and yet in which the image is formed utilizing a single Fresnel lens.
These objectives are achieved by providing an image projection device in which a single positive focal length Fresnel lens in positioned in an optical path of an illuminated image in such a way that light from the image passes twice through the lens, once in each direction, thereby emulating the effect of two positive focal length Fresnel lenses without the cost.
In a preferred embodiment of the invention, a first optical element in the form of a mirror is arranged to reflect light that has been transmitted through the Fresnel lens in a first direction back through the Fresnel lens in a second direction opposite the first direction, and a second optical element is provided for changing a path of light exiting the Fresnel lens in the second direction so that the projected image is formed at a location offset from the path of light from the image source to the Fresnel lens. In this embodiment, the second optical element may be, but is not limited to, a beam splitter arranged to transmit light directly to the Fresnel lens in the first direction, and to reflect light transmitted by said Fresnel lens in the second direction so that the reflected light can be caused to exit the image floater in a third direction transverse to the first and second directions.
Optionally, the image floater of the preferred embodiment may include a background image source positioned such that light from the background image source is transmitted through the beam splitter in the third direction so that the background image is visible together with the projected image. Alternatively, or in addition to the inclusion of a background image, the image projected by the preferred image floater may be combined with additional floating images resulting from cascading of multiple image floaters, i.e., by arranging multiple image floaters in series, so that the projected image from one image floater may be viewed through the beam splitters of subsequent image floaters.